Wide area networking across great distances is a new paradigm of operation for users of telecommunications equipment. The rise of the internet has led to an explosion in the amount of information available to individuals. Whole new industries have formed around the resulting near instantaneous access to information. Working professionals, governments, businesses, and other various segments of the economy are offering new forms of information, services, and products via the new forms of communication. The modern telecommunications devices, such as modems and network servers, allow many new forms of rich information content and are providing more efficient ways of organizing companies and business processes. Essentially, digital telecommunications has proven to be a far more powerful and compelling method of communication than previous, voice only, telecommunications methods. The rise of this new paradigm has led to a huge increase in the digital telecommunications appetite of the consumer.
The problem, however, is that the public switched telephone network through which the majority of this information passes was not designed to handle high bandwidth digital data. The public switched telephone network (PSTN) was primarily designed for point to point voice communication. To connect two users, the network establishes a communications channel between them. This communications channel typically is symmetrical, in that the data transfer rate "upstream" from a user is the same as the data transfer rate "downstream" to the user. Recently, the PSTN has been increasingly used for downloading large amounts of digital data. For voice communication or for relatively small data transfers (e.g., 14 to 33 Kbps), the PSTN functions adequately. For the rich, highly graphical data and modern client-server applications increasingly being used today, however, the PSTN is proving inadequate. Although the PSTN may be able to handle the demands of upstream communications traffic, the downstream traffic has become much greater than the communications channels of the PSTN can efficiently accommodate. Consequently, the vast majority of client server digital communications or high data rate downloads through the PSTN are inefficient and slow. Client-server communications tend to be asymmetric, meaning the largest data transfers are downstream data transfers from the server side of the communications channel (e.g., full motion video files, graphics files, 3D models, and the like) as opposed to the client side. Users are searching for faster connections which will allow them to efficiently download large amounts of digital information from the PSTN.
Prior Art FIG. 1 shows a diagram of a typical client-server communications channel 100 in the PSTN. The client-server communications channel 100 includes a user modem 101 sending transmit data and downloading receive data to and from a user on the left of FIG. 1 (not shown). User modem 101 is coupled to a 2-4 wire converter 102, and in turn, is coupled to a second 2-4 wire converter 103. In communications channel 100, the two wire line connecting 2-4 wire converter 102 and 2-4 wire converter 103 is referred to as the local loop. The 2-4 wire converter 103 is coupled to a PCM (pulse code modulation) codec 104. The 2-4 wire converter 103 and PCM codec 104 are often integrated into one piece of equipment referred to as a subscriber line interface card. PCM codec 104 is coupled to a digital switched network 105 portion of the PSTN, and in turn, is coupled to a internet service provider (ISP) server 106 via a digital subscriber line (e.g., T1 line).
The signals from the user on the left of FIG. 1 to PCM codec 104 are analog signals. The signals from the ISP server on the right of FIG. 1 to the digital switched network are digital signals. The analog signals are converted into corresponding digital signals by the digital to analog converters (DACs) and the analog to digital converters (ADCs) of PCM codec 104.
The 2-4 wire converter 103 and the PCM codec 104 comprise the major portions of a subscriber line interface card. The subscriber line interface card (SLIC) functions by converting the user's analog signal into a digital signal. Once converted, the digital signal is transmitted and switched through the digital switched network 105 of the PSTN. At the destination, the digital signal is coupled directly to ISP server 106 via the digital subscriber line without any intervening codecs.
User modem 101 is designed to receive analog signals. If the analog signal is a voice signal, it is used to drive the speaker of a telephone, recreating the user's voice. If the analog signal is a data signal (e.g., a V.21 modem signal), user modem 101 extracts the digital data from the analog signal for use by the receiving user's computer system. Hence, when ISP server 106 transmits large data files to the user, the digital signals comprising the data files are converted from digital form to analog form by PCM codec 104, transmitted across the local loop, and converted back to digital form by user modem 101. Communications channel 100 functions adequately when used to transmit and receive analog information (e.g., voice). However, communications channel 100 proves inefficient when utilized to transmit high bit rate data streams from ISP server 106.
One of the major bottle-necks to high speed data transmission through a communications channel is the presence of prior art SLICs. For example, one of the primary challenges in designing 56 Kbps PCM type modems is accounting for the detrimental effects of the DACs contained within the SLICs, in combination with the A or .mu. law expansion.
Prior Art FIG. 2 shows a typical prior art SLIC 200. SLIC 200 includes 2-4 wire converter 103 and PCM codec 104. PCM codec 104 includes an 8 bit A law ADC 201 and an 8 bit A law DAC 202. The 8 bit A law ADC 201 transmits information upstream to the digital switched network as a 64 Kbps digital data stream. The 8 bit A law DAC 202 receives downstream information as a 64 Kbps data stream. While SLIC 200 includes A law DAC 202 and A law ADC 201, those skilled in the art understand that alternatively, SLIC 200 could include a .mu. law DAC and .mu. law ADC instead.
As is well known in the art, in the down stream direction (e.g., from an internet service provider to a user), 8 bit A law DAC 202 restricts the number of signaling levels to a theoretical maximum of 256 voltage levels. The number of levels actually available are much lower. Many of the signaling levels around the origin are unusable primarily due to the A law expansion which places these levels too closely together. This forces the user modem 101 to use larger amplitude levels with a corresponding increase in the transmit signal power, as the bit rates increase. Thus, one of the factors limiting the maximum bit rate is the maximum allowable power levels on the local exchange lines. Those desiring more detailed information regarding A law and .mu. law codec standards are directed to "ITU-T G.711, PULSE CODE MODULATION (PCM) OF VOICE FREQUENCIES, International Telecommunications Union" which is incorporated herein as background material.
One solution presently available to the user is to convert to ISDN (integrated services digital network). However, this requires not only a hardware change to the SLIC 200 (e.g., the removal and replacement with a new SLIC) but software and other hardware changes to the PSTN such that the user's local loop is no longer viewed by the PSTN as a regular analog line. Furthermore, modification is often required to the subscriber's local loop on an ad-hoc basis to accommodate the wider bandwidth of an ISDN signal. Consequently, ISDN is presently an expensive option for the user. The installation expense is primarily due to the this lack of a simple, one time hardware change required within the PSTN and the running expense is due to the fact that an ISDN connection requires a total of 144 Kbps (e.g., basic ISDN 2B+D connection) in both directions, compared to only 64 Kbps full duplex for an analog line. Consequently, ISDN imposes more demands on the local exchange's limited available digital throughput to the next level of switching in the PSTN (e.g., digital switched network 105).
Therefore, the expansion of ISDN service at any local exchange has to be carefully planned by the PSTN operators to avoid degrading the quality of service to other normal users of that exchange. The solutions might mean limited ISDN availability and/or the expensive laying of new high bandwidth cable from the local exchange to the next switching level in the PSTN hierarchy. Neither of these solutions is desirable.
Thus, what is required is a system which improves the efficiency of asymmetric data transfers through the PSTN. The required system should improve the downstream data transfer bandwidth for users coupled to the PSTN. The required system should impose minimum changes to the hardware of the PSTN and should be relatively easy to implement in comparison to ISDN. The required system should increase the downstream data transfer efficiency of modems coupled to the PSTN. Additionally, the required system should be compatible with ordinary analog voice communication. The present invention provides a novel solution to the above requirements.